Downhole Heater Assembly and Power Line Communications System

ABSTRACT

A hydrocarbon heating system comprises a surface control panel and a subsurface heating system, connected by an external power cable which also serves as the data communication link between the two parts. The subsurface heating system includes a heating element and two separate, thermally isolated temperature sensors. One thermal sensor, mounted near the heating element, monitors the temperature of the heating element. Another thermal sensor monitors the ambient temperature of fluid in the borehole. The temperature of the heating element may be varied to maintain an optimal well temperature for the production of hydrocarbons. Downhole electronics convert the thermal sensor outputs to digital data, and modulate the digital data onto a power cable that carries power from the surface to the heating element. A control system at the surface controls the heating element via nested control loops monitoring the temperatures of the borehole fluid and the heating element.

This application claims priority to U.S. Provisional Patent Application Ser. No. 61/703,477, titled “Downhole Heater Assembly and Power Line Communications System,” filed Sep. 20, 2012, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF INVENTION

The present invention relates generally to subsurface hydrocarbon extraction, and in particular to a downhole heater assembly and method of temperature communications.

BACKGROUND

Hydrocarbon (e.g., crude oil and natural gas) extraction from subsurface formations is a costly and complex endeavor. After drilling, a well is cased, and tubing is installed in the borehole to carry the target fluids to the surface. If the subsurface pressure is insufficient to flow the oil and gas to the surface, as is the case in many wells that have been in production for some time, a pump is required to pump the fluids out. In many cases, particularly in older wells, the remaining hydrocarbons have such low viscosity, or are clogged with paraffin or other substances, that pumping is inefficient or impossible. In the case of natural gas, iron oxides and other residues may build up and impede the free flow of gas.

It is known in the art to position a heating element downhole, to increase the temperature and hence lower the viscosity of hydrocarbons, melt interfering paraffin, break up concentrations of iron oxides or other residues, and generally facilitate efficiently pumping hydrocarbons to the surface. For example, U.S. Pat. No. 7,363,979 discloses a downhole heater and system of rugged electrical connectors necessary to survive the downhole environment. Proper control of such a downhole heater is essential to optimally heating the downhole fluids. However, control of a downhole heater is complicated by the need to transmit temperature data to the surface for monitoring and control.

The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.

SUMMARY

The following presents a simplified summary of the disclosure in order to provide a basic understanding to those of skill in the art. This summary is not an extensive overview of the disclosure and is not intended to identify key/critical elements of embodiments of the invention or to delineate the scope of the invention. The sole purpose of this summary is to present some concepts disclosed herein in a simplified form as a prelude to the more detailed description that is presented later.

According to one or more embodiments described and claimed herein, a hydrocarbon heating system comprises a surface control panel and a subsurface heating system, connected by an external power cable which also serves as the data communication link between the two parts. The subsurface heating system includes a heating element and two separate, thermally isolated temperature sensors. One thermal sensor, mounted near the heating element, monitors the temperature of the heating element, to ensure that it operates within an optimal temperature range, and to prevent damage from overheating. Another thermal sensor, preferably isolated from the heating element sensor, monitors the ambient temperature in the well. The temperature of the heating element may be varied to maintain an optimal well temperature for the production of hydrocarbons. Downhole electronics convert the thermal sensor outputs to digital data, and modulate the digital data onto a power cable that carries power from the surface to the heating element. A control system at the surface controls the heating element via nested control loops monitoring the temperatures of the borehole fluid and the heating element.

One embodiment relates to a downhole hydrocarbon heating system operative to heat hydrocarbon fluids in an oil or gas well comprising a borehole. The system includes a surface control panel and a subsurface heating system operative to be deployed down the borehole. The subsurface heating system includes a heating element; a first thermal sensor operative to sense the temperature of the heating element; and a second thermal sensor operative to sense the ambient temperature of fluid in the borehole. The system further includes a power cable connecting the surface control panel and the subsurface heating system. The power cable is operative to selectively carry power from the surface control panel to the heating element. The power cable is further operative to carry temperature data from the first and second thermal sensors to the surface control panel.

Another embodiment relates to a method of controlling the temperature of hydrocarbons in a well comprising a borehole. Disposed in the borehole is a subsurface heating system including a heating element, a first thermal sensor operative to sense the temperature of the heating element, and a second thermal sensor operative to sense the ambient temperature of fluid in the borehole. The temperature of the heating element and the ambient temperature of fluid in the borehole are monitored. Power is applied to the heating element in response to the sensed ambient temperature of fluid in the borehole being below a lower limit, and the sensed temperature of the heating element being below a setpoint. Power is removed from the heating element in response to the sensed ambient temperature of fluid in the borehole being above an upper limit or between the lower and upper limit, or the sensed temperature of the heating element being above the setpoint.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

FIG. 1 is a sectional diagram of a producing oil well with a downhole heating and control system according to one embodiment of the present invention.

FIG. 2 is a more detailed sectional view of the downhole heating and control system according to one embodiment of the present invention.

FIG. 3 is a functional block diagram of a temperature sensor control and communication system according to one embodiment of the invention.

FIG. 4 is a flow diagram of a method of controlling the temperature of hydrocarbons in a well.

DETAILED DESCRIPTION

It should be understood at the outset that although illustrative implementations of one or more embodiments of the present disclosure are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents.

FIG. 1 depicts a modern, mature, production oil well the well 10 comprises a borehole 12 in which production tubing 14 has been installed. A pump jack 16 creates a pressure differential that pulls hydrocarbons from a subsurface production zone 18 into the borehole 12. The hydrocarbons enter the production tubing 14 through a perforated section 20, and are pumped to the surface. In many cases, the hydrocarbon fluids are of low viscosity, clogged with paraffin or iron oxides, or otherwise present obstructions to efficient pumping. In these cases, a hydrocarbon heating system 8 according to one or more embodiments of the present invention may be deployed to heat the fluids. The hydrocarbon heating system 8 comprises two parts: a surface control panel 58 and a subsurface heating system 22, connected by an external power cable 54 which also serves as the data communication link between the two parts.

The subsurface heating system 22, depicted in greater detail in FIG. 2, is attached to the downhole side of production tubing 14, below the perforated section 20. The core of the subsurface heating system 22 is a heating element 24. The heating element 24 is a long, thin element, e.g., in one embodiment, approximately ten feet in length and less than one inch in diameter. The heating element 24 is disposed in a lower housing 26, such as a section of 2.5 inch stainless steel pipe. The heating element 24 is generally centered within the lower housing 26, and is preferably held in position by thermally conductive cement 28, such as ceramic heat transfer cement. In operation, the lower housing 26 is disposed in the borehole 12 at a target depth corresponding to the subsurface production zone 18, to heat the hydrocarbon fluids. In one embodiment, upper and lower standoffs 30 welded to the exterior of lower housing 26 maintain the lower housing 26 generally centered within the borehole 12. In one embodiment, the lower housing 26 is approximately ten feet in length.

One example of a suitable heating element 24 is a 12 kW Maxizone heater, available from Chromalox of Pittsburgh, Pa. The heating element 24 preferably operates in the range of 500-600 degrees Fahrenheit (F). The heating element 24 is powered, in one embodiment, by a three-phase, 12-gauge, Teflon stranded internal power cable 32 carrying, e.g., 480 VAC, 3-phase power obtained from the surface control panel 56 via external power cable 54. A first thermal sensor 34, such as a K type thermocouple, is embedded in, or disposed proximate to, the heating element 24. The first thermal sensor 34 monitors the temperature of the heating element 24.

In one embodiment, an insulating section 36 separates the lower housing 26 from the upper housing 38. The insulating section 36 ensures separation between the heating element 24 and control electronics and a second sensor 40 housed in the upper housing 38. In one embodiment, the insulating section 38 is in the range of eight to ten feet long. This distance allows the second sensor 40 to monitor the ambient temperature of fluids in the borehole 12, without undue influence from the heating element 24. The internal power cable 32 passes through the insulating section 36.

An upper housing 38, disposed opposite the insulating section 36 from the lower housing 26, includes downhole printed circuit board (PCB) 42, which contains the downhole control electronics. The downhole PCB 42 is powered from the internal power cable 32 via a transformer 44. A second thermal sensor 40 is disposed in the upper housing 38, and connected to the downhole PCB 42. The second thermal sensor 40 is operative to monitor the ambient temperature of fluids in the borehole 12, which are preferably maintained in the range of 150-200 degrees F., and more preferably in the range of 160-180 degrees F. The transformer 44, PCB 42, second thermal sensor 40, and power cable 32 are preferably encased in an inert compound 46. The transformer 44 and PCB 42 may comprise any suitable components—for example, components designed and certified for use in harsh environments. In one embodiment, separation of the second thermal sensor 40 from the heating element 24 is provided, not by a separate insulating section 36, but by locating the second thermal sensor 40 in the upper region of the upper housing 38. In this embodiment, the upper housing 38 is preferably eight to ten feet long.

Above the upper housing 38, a cable admission connector 48, connected thereto by a coupler 47, includes a port 50, to which is attached a connector 52. The connector 52 provides a quick-detach connection between the internal power cable 32 providing power to the PCB 42 and heating element 24, and an external power cable 54 that extends up the borehole 12 external to the production tubing 14. The external power cable 54 is preferably an armored cable designed for operation in the harsh environment of the borehole 12. One example of a suitable external power cable 54 is a 3×, 4 AWG flat armored cable available from Schlumberger of Houston, Tex.

The cable admission connector 48 connects to the upper housing 38, via a coupler 47, in a manner that excludes borehole fluids from the interior of the upper housing 38. Indeed, from the coupler 47 downward, the system 22 is a sealed environment, with no incursion of borehole 12 fluids into the interior of the upper housing 38, insulating section 36, or lower housing 26. One example of a suitable cable admission connector 48 is the ZPTC series connector available from Taurus Engineering of Long Beach, Calif. In one embodiment, the power cable 32 also carries temperature monitoring signaling from the downhole thermal sensors 34, 40 to the surface, as described further herein.

Above the cable admission connector 48—which constitutes the upper extent of the subsurface heating system 22—is a perforated tubing section 20. The perforated tubing section 20 is the lower-most portion of the production tubing 14, and may be, for example, four feet long. Perforations in the perforated tubing section 20 allow hydrocarbon fluids to enter the production tubing 14, where they are pumped to the surface by the pump jack 16. Intermittently, such as at intervals of 30 feet, straps 56 secure the exterior power cable 54 to the production tubing 14. The straps may be applied, as required or desired, as the subsurface heating system 22 and production tubing 14 are lowered into the borehole 12. At the surface, the external power cable 54 enters a control panel 56, which houses the surface electronics.

In general, the sections of the subsurface heating system 22—such as the lower housing 26, insulating section 36, and upper housing 38—may be constructed from 2.5 inch steel pipe with National Pipe Thread Taper (NPT), coupled together by 2 inch×2.5 inch NPT couplers, as well known in the drilling and hydrocarbon production arts.

The first thermal sensor 34 monitors the temperature of the heating element 24, and the second thermal sensor 40 monitors the ambient borehole fluid temperature. Both thermal sensors 34, 40 are connected to the downhole PCB 42, which converts the thermal sensor 34, 40 voltages to digital temperature data, and transmits the temperature data to the surface control panel 56 using power line communications.

FIG. 3 presents a functional block diagram of the electrical and control system of the hydrocarbon heating system 8. The subsurface heating system 22 comprises the heating element 24, first thermal sensor 34 monitoring the heating element 24, second thermal sensor 40 monitoring the ambient borehole fluid temperature, and the downhole PCB 42. The PCB 42 is powered by a transformer 44 connected to the internal power cable 32. The PCB also connects across two phases of the internal power cable 32 to communicate temperature data to the surface control panel 58 via power line communications. The coupling across two phases obviates the need for a neutral, or ground, conductor.

The PCB 42 includes a power line controller operative to modulate digital data on a power line for communication to the surface control panel 58. One example of a suitable power line controller is a PL3120 power line smart transceiver, available from Eschelon Corp. of San Jose, Calif. The PL3120 includes a power line transceiver, a Neuron processor core, embedded memory, and associated circuits. The PCB 42 additionally includes two Analog to Digital Converters (ADC) to convert the output voltages of the thermal sensors 34, 40 to digital values. One example of a suitable ADC is a MAX31855 cold-junction compensated thermocouple-to-digital converter, available from Maxim Integrated Products of San Jose, Calif. The ADCs communicate with the power line controller, such as via an I2C interface.

In one embodiment, the thermal sensor 34, 40 values are converted to Lonworks protocol Standard Network Variable Type (SNVT) temperature values. As known in the art, Lonworks is an industry standard networking protocol targeted to address the needs of control applications. The Lonworks protocol defines a plurality of SNVTs for common control system variable types. For example, the temperature SNVT defines a number between zero and 65535 that corresponds to a temperature between −247 and 6279.5 degrees Celsius. The Lonworks protocol also defines the standards for power line communications, by which digital data is modulated onto an AC power line.

The surface control panel 56 includes also includes a power line controller 60 operative to extract thermal 34, 40 data from the external power cable 54. The surface power line controller 60 be the same as, or similar to, the power line controller on the downhole PCB 42. In one embodiment, the surface power line controller 60 converts the temperature data from the subsurface heating system 22 into two 4-20 ma current loop signals for use as feedback signals for two nested temperature control loops, controlled by a temperature controller 62. An inner control loop prevents the heating element 24 from operating beyond a maximum setpoint, such as 600 degrees F. An outer control loop modulates power to the heating element 24 to maintain a desired ambient downhole fluid temperature, such as in the range of 150-200 degrees F., and more preferably in the range of 160-180 degrees F. The temperature controller 62 controls the downhole temperatures by modulating the 480 VAC, 3-phase power to the heating element 24, such via a solid state 3-phase relay. In one embodiment, regardless of the control loop outputs, downhole power is periodically applied for a brief duration—such as for two seconds every thirty seconds—to allow the subsurface heating system 22 to obtain and transmit updated temperature readings. One example of a suitable temperature controller 62 is an EZ-Zone PM controller, available from Watlow Electric Manufacturing Co. of St. Louis, Mo.

FIG. 4 depicts a method 100 of controlling the temperature of hydrocarbons in a well. Although those of skill in the art will appreciate that the method describes a continuous and ongoing process, for the purpose of explanation it may be said to “begin” by monitoring the temperature of the heating element 24, via the first thermal sensor 34, and the ambient borehole fluid temperature, via the second thermal sensor 40 (block 102). The ambient borehole fluid temperature is controlled in an outer control loop (block 104). If the ambient temperature is within a predetermined range, such as 150-200 degrees F., or more preferably 160-180 degrees F., or if the ambient temperature exceeds the maximum desired temperature, the heating element 24 is not energized, and the system 8 continues to monitor the downhole temperatures (block 104). If the ambient borehole fluid temperature falls below the desired minimum temperature, then the heating element 24 is energized to further heat the hydrocarbons, subject to the operation of the inner control loop. If the temperature of the heating element 24 is at or below a predetermined setpoint (block 106), then the heating element 24 is energized by supplying 480 VAC, 3-phase power via the external power cable 54 and internal power cable 32 (block 108). However, if the temperature of the heating element 24 exceeds its setpoint (block 106), then the heating element 24 is not energized, regardless of the ambient borehole fluid temperature, to prevent damage to the heating element 24. In either case, the system 8 then continues to monitor the downhole temperatures (block 102).

Those of skill in the art will recognize that the method 100 as depicted in FIG. 4 is representative only, and many variations are possible within the scope of the present invention. For example, in one embodiment, once the ambient borehole fluid temperature is within the desired range, a partial current may be supplied to the heating element 24 to maintain its temperature, with the power being removed only if the first thermal sensor 34 indicates a temperature above the setpoint. As another example, as discussed above, even when the heating element 24 is not energized by the control loops (i.e., method block 108 is not reached due to the downhole temperatures reported), the power may be applied periodically for brief durations (e.g., two seconds every thirty seconds) to energize the downhole electronics and obtain updated temperature readings. Furthermore, various timers may be implemented to halt the method 100 in a state (e.g., block 108) for predetermined durations. In general, those of skill in the art may devise numerous temperature control schemes, given the teachings of the present disclosure and the resources of two separate, independent thermal sensors 34, 40 reporting both heating element 24 and ambient borehole fluid temperatures, respectively. All such heating element 24 control schemes would fall within the broad scope of the present invention.

The hydrocarbon heating system 8 of the present invention presents numerous advantages over the prior art. By providing two temperature sensors 34, 40 in the subsurface system 22, two critical temperatures may be monitored. By controlling the heating element 24 in a nested control loop, the ambient borehole fluid temperature may be regulated as desired, subject to the protective monitoring and control function of limiting the temperature of the heating element 24. By employing power line communications to convey temperature readings from the subsurface thermal sensors 34, 40 to the surface, the need for expensive and fragile dedicated communication lines is obviated. This improves both lowers cost and improves the reliability of operation in the hostile environment of the borehole 12.

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. 

What is claimed is:
 1. A downhole hydrocarbon heating system operative to heat hydrocarbon fluids in an oil or gas well comprising a borehole, the system comprising: a surface control panel; a subsurface heating system operative to be deployed down the borehole and including a heating element; a first thermal sensor operative to sense the temperature of the heating element; and a second thermal sensor operative to sense the ambient temperature of fluid in the borehole; and a power cable connecting the surface control panel and the subsurface heating system, the power cable operative to selectively carry power from the surface control panel to the heating element, and further operative to carry temperature data from the first and second thermal sensors to the surface control panel.
 2. The system of claim 1 wherein the heating element and the first thermal sensor are disposed in a lower housing operative to be deployed in the borehole at the depth of a hydrocarbon production zone.
 3. The system of claim 1 wherein the heating element is disposed centrally in the lower housing and secured by a thermally conductive concrete.
 4. The system of claim 1 wherein the lower housing comprises standoff elements secured to the exterior thereof in radially spaced positions, the standoff elements operative to maintain the lower housing generally centered in the borehole.
 5. The system of claim 1 wherein the second thermal sensor is disposed in a spaced relationship with the heating element such that the temperature sensed by the second thermal sensor is not directly influenced by the heating element.
 6. The system of claim 5 wherein the subsurface heating system further comprises an insulating section interposed between the heating element and the second thermal sensor.
 7. The system of claim 1 wherein the subsurface heating system further comprises a downhole printed circuit board (PCB) containing electronics to convert the first and second thermal sensor outputs to digital data, and to modulate the digital temperature data onto the power cable.
 8. The system of claim 7 further comprising a transformer operative to tap power from the power cable and to power the downhole PCB.
 9. The system of claim 7 wherein power cable carries 3-phase power, and wherein the electronics on the downhole PCB are operative to modulate digital data across two phases of the power cable.
 10. The system of claim 1 wherein the subsurface heating system is deployed below production tubing operative to extract hydrocarbons from the well.
 11. The system of claim 1 wherein the surface control panel comprises: a power line controller operative to receive sensed temperature data from the power cable; and a temperature controller operative to control power to the heating element by selectively connecting the power cable to a power source in response to the sensed temperature data received from the power line controller.
 12. The system of claim 10 wherein the temperature controller is operative to control power to the heating element by: in an outer control loop, applying power to the heating element in response to the ambient temperature of fluid in the borehole being below a predetermined lower limit; and in an inner control loop, applying power to the heating element only if the temperature of the heating element is at or below a predetermined setpoint, without regard to the outer control loop.
 13. The system of claim 10 wherein the temperature controller is operative to control power to the heating element by periodically applying power to the heating element for brief durations without regard to the ambient fluid or heating element temperature values.
 14. A method of controlling the temperature of hydrocarbons in a well comprising a borehole, in which is disposed a subsurface heating system including a heating element, a first thermal sensor operative to sense the temperature of the heating element, and a second thermal sensor operative to sense the ambient temperature of fluid in the borehole, the method comprising: monitoring the temperature of the heating element and the ambient temperature of fluid in the borehole; applying power to the heating element in response to the sensed ambient temperature of fluid in the borehole being below a lower limit, and the sensed temperature of the heating element being below a setpoint; and removing power from the heating element in response to the sensed ambient temperature of fluid in the borehole being above an upper limit or between the lower and upper limit, or the sensed temperature of the heating element being above the setpoint.
 15. The method of claim 11, further comprising periodically applying power to the heating element for brief durations without regard to the values sensed by the first and second thermal sensors.
 16. The method of claim 12 wherein periodically applying power to the heating element for brief durations comprises applying power to the heating element for two seconds every thirty seconds. 